The most common modes of diagnostic ultrasound imaging include B- and M-modes (used to image internal, physical structure), Doppler, and color flow (the latter two primarily used to image flow characteristics, such as in blood vessels). In conventional B-mode imaging, ultrasound scanners create images in which the brightness of a pixel is based on intensity of the echo return, i.e., the amplitude of the reflected waves is employed to produce black and white images of the tissues.
Two-dimensional ultrasound images are often hard to interpret due to inability of the observer to visualize the two-dimensional representation of the anatomy being scanned. However, if the ultrasound probe is swept over an area of interest and two-dimensional images are accumulated to form a three-dimensional volume, the anatomy becomes much easier to visualize for both the trained and untrained observer.
B-mode ultrasound imaging suffers from the inherent imaging artifact referred to as speckle. Speckle is the mottling found in the images produced from interference patterns of multiple receive echoes. This mottling is primarily caused by nulls in the acoustic interference pattern, but other anomalies in the image, e.g., random electronic noise, can cause mottling. The acoustic nulls are accentuated by the log compression required to display the full dynamic range of the ultrasound image. These nulls appear as black holes in the image. Speckle noise and artifacts limit the range of acceptable view angles in three-dimensional ultrasound imaging.
Since any parameter which changes the sum of the returning echoes will alter the speckle pattern, a number of conventional methods exist for reducing the speckle image artifact. Examples of such conventional methods include multiple transmit focusing, spatial compounding, frequency compounding and spatial low-pass filtering. The multiple transmit focusing, spatial compounding and frequency compounding techniques suffer from reduced frame rates, whereas spatial low-pass filtering has reduced resolution.